In applications requiring rapid scanning of an illumination beam, such as barcode scanning, one method commonly employed for rapidly and repetitively scanning the illumination beam across a scanned region is mirror dithering. Dithering, i.e. rapid rotational oscillation of an illumination beam steering mirror about an axis substantially parallel to the mirror face, causes the illumination beam to move rapidly back and forth generating a scan line. When this scan line illuminates a barcode, the resulting time dependent signal due to detected light scattered and/or reflected from the bars and spaces of the barcode is decoded to extract the information encoded therein. To be used in such scanning applications, the dithering motor generating the mirror motion must be stable and typically employs some sort of feedback between the motor and the motion of the mirror. Particularly for handheld scanning applications, the dithering assembly should be light, compact, reliable, and consume minimum power while producing sufficiently large amplitude motion for scanning. Scanners are typically constructed with a feedback control circuit to actively adjust the length of the scan line so as to remain substantially constant.
Previous dithering assemblies have typically comprised a pair of magnets and a pair of magnetic coils. FIG. 1 illustrates a dithering assembly 100 comprising a mirror/magnet assembly 110, drive coil 106, feedback coil 108, bending member 112, and mounting member 114. The mirror/magnet assembly 110 comprises mirror 102, mirror bracket 103, and drive magnet 104 and feedback magnet 105. The drive coil 106, feedback coil 108 and mounting member 114 may be part of or mounted within a housing (not shown) for dithering assembly 100. The bracket 103 holds mirror 102 and is connected to mounting member 114 by bending member 112, which may comprise a thin, flat sheet of flexible material which acts as a bendable spring. Bending of member 112 results in rotation of mirror/magnet assembly 110 about an axis substantially parallel to mirror 102, perpendicular to the plane of FIG. 1.
It has been generally thought to be advantageous to locate the axis, i.e., the center of rotation (COR), coincident with the center of gravity (COG) of mirror/magnet assembly 110. To protect the ditherer in the presence of severe mechanical shock, a means to constrain the motion of the center axis of rotation may be employed which is convenient in that, at this point, there is no lateral motion (i.e., motion in the plane perpendicular to the COR axis). For example, such means may comprise a moving pin, whose axis is the same as the COR axis, rotating in a stationary hole. The pin does not touch the inside of the hole in normal operation, since this touching would dampen the motion of the ditherer and reduce efficiency. Since there is no lateral motion of the pin--it merely rotates about its axis--the required clearance inside the stationary hole need only be sufficient to accommodate process and temperature variations. Under shock, the pin functions to restrain movement of the COR. If the COR and the COG are the same, and movement of that point is constrained, then shock along any rectilinear axis will only translate the pin to the inside of the hole--no rotation will occur. Thus, no additional shock constraint features are necessary. The present inventors have recognized that if the COR and the COG are not coincident, rotational motion need be restrained in the normal dithering direction.
The dithering assembly 100 comprises an oscillating structure which has a resonant frequency determined by the effective spring constant of bending member 112 and the effective mass of the mirror/magnet assembly 110 and any components attached thereto. The motion of mirror/magnet assembly 110 is driven by passing an oscillating drive current through drive coil 106 thereby generating an oscillating magnetic driving force on drive magnet 104. The maximum amplitude of dithering motion of the mirror 102 occurs when the drive current oscillates at the resonant frequency of dithering assembly 100, i.e., when the dithering assembly 100 is driven resonantly. It is important to drive the dithering assembly 100 resonantly to obtain the maximum dithering amplitude with minimum drive power consumption. It is also important that the position and length of the resulting scan line remain constant.
Even when feedback is employed to keep the drive frequency matched to the resonant frequency, there still can be considerable variation in the amplitude and position of the resulting dithering motion. These amplitude variations may result from a variety of manufacturing and operational variables which may be difficult to control, including but not limited to the precise mass of mirror/magnet assembly 110 and any components attached thereto, the precise dimensions and force constant of bending member 112, the temperature, wear of the dithering assembly, and/or the spatial orientation of the moving drive magnet with respect to the drive coil. Since the amplitude of the dithering motion determines the position and length of the scan line produced by the dithering assembly, and since it is important for the position and length of the scan line to be constant for proper operation of the barcode scanner, the amplitude variations of the dithering motion must be minimized. Such amplitude variations may be minimized by using position feedback to control the amplitude of the drive force. However, such feedback necessitates additional sensing and control electronics, and adds to the overall power consumption, cost, and/or complexity of the barcode scanner. Furthermore, optimization of such a feedback system for proper operation may depend on the same variables which cause the amplitude fluctuations in the first place.
FIG. 2 illustrates typical waveforms for position, velocity, and drive force for a resonantly driven dithering assembly. Position waveform 152 and velocity waveform 154 are substantially sinusoidal, with a phase shift of 90 degrees between the position and the velocity. For a dithering assembly driven at its resonant frequency, velocity waveform 154 will be in phase with drive force waveform 156. Drive force waveform 156 is shown as a square wave in FIG. 2, but may also comprise a substantially sinusoidal waveform.
This feedback has been accomplished in previous dithering assemblies by velocity feedback. Feedback coil 108 experiences an oscillating magnetic field due to feedback magnet 105, which is attached to bracket 103. The electrical potential developed across feedback coil 108 varies directly with time derivative of the magnetic flux at feedback coil 108, and hence with the velocity of feedback magnet 105 and dithering mirror 102. The zero crossings of the feedback potential, which occur when the mirror velocity is zero, are used to trigger switching of the polarity of the drive current in drive coil 106, thereby reversing the drive force exerted on drive magnet 104 and mirror 102. In this manner, the switching frequency of the drive force is always locked to the frequency of the dithering motion of dithering assembly 100 and the drive force is in phase with the velocity as required for a resonantly driven system.
There are several weaknesses with the feedback scheme described above. The electrical potentials developed across feedback coil 108 are typically quite small, on the order of a few millivolts. These signals must be amplified for use as a feedback signal, and the resulting feedback signal is quite noisy. There may be significant cross talk between the drive magnetic fields and feedback coil 108 because the drive coil 106 and drive magnet 104 are nearby. Further, since the feedback coil 108 is manufactured by making many turns of very fine gauge wire in order to maximize output voltage, it is therefore difficult to manufacture and thus can be expensive, bulky, relatively delicate, and/or unreliable.